I don't believe it is based on a 365 day year...I think it based off of 360 day year.

(0.15 / 360) * 20 =  0.00833 (or 0.833%)

So basically, assuming it minted immediately, your return would be .833%, every 20 days.
Another way to look at it is 1.25% per month.

(Keep in mind actual results will vary, slightly due to timing of blocks etc., but on average it will be pretty close to this)

(Also keep in mind, that in less than a year probably about 9 months (ish) the coin will drop to the 10% annual rate)